An opto-mechanical system configured for use in a bar code scanner is illustrated in FIG. 1. As shown, the system comprises light source 1 (typically a laser), focusing optics 2, fold mirror 2a, and scanning mirror 3. The system may also include folding and/or pattern generation mirrors (not shown). Light beam 5 is emitted from light source 1, whereupon it is directed to travel along an optical path. Focusing optics 2 are placed along the optical path, and are utilized to focus the beam 5, to provide a minimum diameter at point 6, known as the beam waist. A scanning mirror 3 (illustrated as a facet wheel which is typically in the shape of a polygon) is placed along the optical path, and caused to rotate by a motor (not shown) around center of rotation point 3a. The light source 1, focusing optics 2, and scanning mirror 3 are typically placed in the interior of a housing having a window 4, through which the beam is directed to emerge from the housing. The window 4 is configured with appropriate transmission characteristics to enable the beam 5 to exit the interior of the housing with minimal loss.
In operation, the scanner is aimed towards a target (e.g., a bar code symbol), with the scanning mirror 3 (and pattern generation mirrors if present) operating to scan the beam 5 over the target in a predetermined pattern. A collection subsystem (not shown) which typically includes a photodetector, then collects at least some of the light reflected or scattered off the target, and the photodetector produces an analog signal having an amplitude determined by the intensity of the collected light. Since more light reflects off lighter areas than dark areas (or elements), the amplitude of the analog signal will vary accordingly as the beam 5 scans the target.
The scanner also includes a signal processing subsystem (not shown) for digitizing the analog signal and providing a digital signal, where the width of and spacing between the pulses in the digital signal corresponds to the width of the dark and light areas (e.g., bars/spaces or elements) making up the target. The digital signal is then decoded by a decoder (not shown), either located within the scanner, or located externally to it.
As shown, the outer bounds of the scan volume, identified with numerals 7 and 8 in FIG. 1, are typically situated on either side of the beam waist 6.
One reason why there are limits to the scan volume is, as illustrated in the figure, that on either side of the beam waist, along the beam axis 6a, e.g., at locations 6' and 6" in FIG. 1, the beam diameter, or spot size, increases as a function of the distance from the beam waist 6. This is due to fundamental diffraction effects. As the beam diameter increases, the ability to successfully read targets, especially those with closely spaced elements, diminishes, and eventually is lost. Performance at the outer extremes of the scan volume is further limited by collected optical power, which decreases approximately as the square of the distance from the scanner. Together these factors determine the outer bounds of the scan volume.
The limitation due to spot size can be explained with reference to FIGS. 2a-2b. FIG. 2a illustrates an analog signal 10 which results from passing a laser beam 9 across a target 11 (in this example a bar code symbol) in the indicated scanning direction. As shown in FIG. 2d, the diameter of the beam 9 is no larger than the smallest dark or light element in the target symbol (identified with width t.sub.1). Accordingly, the amplitude modulation depth of the portion of the analog signal which derives from the more closely spaced elements 12a, identified with reference numeral 12b in the figure, is about the same as the amplitude modulation depth of the portion of the analog signal which derives from the less closely spaced elements 13a, identified with reference numeral 13b in the figure.
FIG. 2b illustrates the effect of scanning the same target with a beam 9' of larger diameter. FIGS. 2c and 2e illustrate the relationship of this larger beam 9' to the width of the narrower (t.sub.1) and wider (t.sub.2) bars in the example. As illustrated, the diameter of the beam 9' is larger than the width of the more closely spaced elements 12a' making up the target.
The amplitude modulation depth of the lower portion 12b' of the analog signal which derives from the more closely spaced elements 12a' is less than that of the higher portion 13b' of the analog signal which derives from the less closely spaced elements 13a'. This effect occurs since the beam 9' is too large to wholly fit within the confines of the more-closely spaced elements 12a', but not the less-closely spaced elements 13a'. Since the total energy contained in both the large and small spots is the same, the amplitude modulation depth of the analog signal will be determined by the amount of overlap between a spot and an element: Comparing three situations, when a narrow element 12a' is encountered by a larger spot 9', a situation illustrated in FIG. 2c, the amount of overlap (indicated by cross-hatching in the figure) is less than the amount of overlap between a smaller spot 9 and a narrow element 12a, a situation illustrated in FIG. 2d, and the amount of overlap between a larger spot 9' and a wider element 13a', a situation illustrated in FIG. 2e. Therefore, the amplitude modulation depth of the analog signal in the first situation will be less than that for the latter two situations.
Because of the reduced amplitude modulation depth of that portion of the analog signal which derives from the more-closely spaced elements 12a' (reference 12b' in FIG. 2b), it will be more difficult for the signal processing subsystem to distinguish that portion of the signal from noise due to ambient light and the like, and, therefore, to correctly digitize that portion of the analog signal. As the distance from the beam waist 6, and therefore the beam diameter, increases, eventually a point will be reached on either side of the beam waist 6 where the signal processing subsystem cannot successfully digitize that portion of the analog signal derived from the more-closely spaced bars.
Although attempts have been made to extend the limits of the scan volume, frequent drawbacks have been encountered. In European Publication No. 0 433 593, recognizing that the lower amplitude portion 12b' of the analog signal (as illustrated in FIG. 2b) will have a higher frequency than the higher amplitude portion 13b', a variable gain amplifier is utilized to selectively amplify the lower amplitude portion 12b' of the analog signal. The objective is to compensate for the effects of the larger spot size by configuring the amplifier to have a frequency response which has a greater gain over higher frequencies than at lower frequencies. This filter is utilized to amplify the lower amplitude portion 12b' of the analog signal until it is about equal to that of the higher amplitude portion 13b'.
One problem with this approach, as further described in the next section, is that the filter cannot be configured to properly compensate for the effects of large spot size at all points within the scan volume, since it will overcompensate or undercompensate for the effects of the large spot size at some points within the scan volume. A solution to this problem, proposed in European Publication 0 433 593, is to make the equalizing filter adapt to changes in detected frequency or target position. However, the hardware required to provide this feedback information is bulky and expensive, and the information itself is typically unreliable in high-throughput scanners where the target may only be scanned once as it passes through the scan volume. Another problem, also further described in the next section, is that the filter may have an asymmetrical impulse response, which results in alteration of the relative position of detected edges.
Another approach to increasing depth-of-field is the use of an optical element called an axicon to keep the beam diameter small over the desired range of distances, as described in U.S. Pat. No. 5,080,456. However, some of the disadvantages are that this approach is optically inefficient, does not average out paper noise, and does not inherently provide constant impulse response, the benefits of which will be described later.
Consequently, it is an object of the present invention to provide an optical scanning system and related method which overcomes these disadvantages.